I have the same issue. The images I place into OmniGraffle via Place > "Link to Image File" do not update when I modify and save them.

Some background: I essentially want to take individual omnigraffle canvases and compose them into a larger process flow. But I want the content in the process flow to update when I update the relevant canvases. Is there a better way to do this given that I'm working with OmniGraffle files exclusively?